Bright copper plating process



United States Patent 7 2,955,992 BRIGHT COPPER PLATHI G PROCESS ErichLaue, Watertown, Conu., assignor to MacDermid Incorporated, Waterbury,Conn., a corporation of Connecticut No Drawing. Filed Aug. 8, 1957, Ser.No. 676,950

10 Claims. (Cl. 204-52) This invention relates to the electroplating ofbright copper from copper cyanide solutions. More specifically itrelates to a new and improved process for plating from a bath containing.a particular brightening agent with the use of a modified current.

In recent years selenium brighteners have been introduced into brightcopper plating since they permit operation over a wide range of currentdensities, and the low valence selenium compounds are especiallyefficient brighteners, being active at very small concentramay consistgenerally of metals, such as antimony or' lead. However, the use of asecondary brightener complicates the process since the bath now becomescritical, it being difiicult to maintain the two brighteners in properbalance. Furthermore, these secondary brighteners produce considerablesurface irregularity or roughness in the plated copper.

In bright plating it is generally accepted that the application ofperiodically reversed current to the cell gives smoother and moreuniform deposits than uninterrupted direct current. This generalproposition does not hold in the case of copper cyanide baths containinglow valence selenium brighteners either alone or in combination withsecondary brighteners. For example, a typical decor-ative cycle of 15seconds plate and 3 seconds reversed current definitely increases themedium current density dullness, while longer and more sacrificialcycles are still worse. The prior art indicates that the quantity ofreversed current should equal more than at least 10% of the coulombs ofthe plating phase, and in the above example the quantity 'of reversedcurrent was 20% of of that of the plating current.

It is an object of the present invention to provide a broad currentdensity range bright copper plating process which requires only onebrightener, while using either interrupted current or periodicallyreversed current. It is a further object of the present invention toprovide a more economical plating process and one which is simpler tocontrol. It is a further object to provide a plating process whichpermits high plating speeds and in which the resulting copper plate isbright, ductile and adherent over a wide range of plating thicknesses.It is a still further object of the present invention to provide aprocess which considerably reduces the roughness of the copper plate,and also to provide a process which ice on the work is prevented byinterrupting the plating current, either by periodic currentinterruption or by periodic current reversal, with reverse phases soshort that they merely eliminate polarization. I have also found that byso operating, the troublesome secondary metallic brighteners, with theirinherent control and product quality problems, are unnecessary.

The preferred brighteners in accordance with the present process arethose selenosulfates (SeSO and selenocyanates (SeCN)- soluble in thecopper plating bath, and preferably the alkali metal salts of thesematerials. Both the selenosulfates and selenocyanates are stable indilute solutions and are preferably added to the bath from solutions ofsuitable concentration, which makes for easy dosage and gooddistribution of the brightener throughout the bath. 7

In addition to the selenosulfates and selenocyanates, many otherselenium compounds soluble in the copper cyanide bath give similarefiects'. The selenium compounds within the scope of the presentinvention may be either organic or inorganic, but they will all containselenium with a valence lower than +4, which are referred to herein aslow valence selenium compounds. In the selenides for example seleniumhas a valence of 2. In other cases covalent bonds seem to be involvedand the question of valence cannot be answered simply. Examples of suchcompounds are the selenosulfates, selenocyanates, polyselenides, andselenium compounds corresponding to the dithionites and dithionates. Forthe purposes of the present invention the designation selenium compoundswith selenium valencies lower than +4 is sufficient since the compoundsin this class have been found to behave relatively similarly. However,unfavorable qualitative and quantitative differences were observed whereselenium compounds of valence +4 and higher, for example the selenitesand selenates, were employed. The present invention also contemplatesthe use of finely divided metallic selenium, which I believe forms lowvalency selenium compounds in the bath. 7

Extensive tests have shown that the brightening agents of the presentinvention give excellent results at extremely low concentrations in theplating bath. Furthermore, limitation of polarization by the methods setforth hereinafter not only produces a uniform brightness over the wholeplating current density range, but also by so operating the brightenerconcentration is much less critical than it is with uninterrupted directcurrent. The present process gives a bright plate of excellentuniformity with selenium concentrations equivalent to 0.00008 to 0.0032gram of selenium per liter. The activity of the brighteners actuallybegins about at 0.000008 gram per liter, and with some of the organiccompounds as much as 0.05 gram per liter is required.

It is believed that the current cycles of the present process have theefiect of preventing too high cathodic polarization. If interruptedcurrent is used, the plating phase should not be too long, preferablyshorter than 60 seconds. The shortest phases possible with mechanicalswitches, which generally will be about 1 second, work quite well. Inthis connection it has been found that unfiltered half wave current froma rectifier, which pro- "duces a cycle of 5 second plate: secondinterrup has improved. tolerances toward impurities, especially I lowvalence selenium brightener the medium current density dullness iseliminated if too high polarization tion, does not produce the desiredeilect. Thus the plating phase should not be less than about 1 second.The length of the interruption phase must be suflicient to permit aconsiderable decrease of cathodic polarization. If,

on the other hand, this phase is made exceedingly long,

plating speed is of course reduced unnecessarily. The optimum length ofinterruption depends on various conditions, such as temperature anddegree of agitation. It has been found that interruption times amountingto from 10,to of the plating time are satisfactory,

Such cycles as :1 seconds or 10.4 produce very satisfactory plates as docycles like 15:3 and 40:16.

When the current is periodically reversed, the reversed phase must be ofa duration giving satisfactory decrease of polarization without leadingto substantial deplating action. In other words, operating conditions ofthe present process with periodically reversed current are outside theoperating conditions of normal periodically reversed plating. Inaccordance with the present invention, I have found that the reversedphase must apply less than of the coulombs of the plating phase, andunder these conditions undesirable reactions at the work are avoided.Generally, for convenience sake the reversed current is applied at thesame current density as the plating current. Cycles such as /2 secondsor :1 are preferred, but 40:2 still produces satisfactory plate. Thetimes above mentioned apply to the current on the work, and if theswitching devices used work with considerable delay this time lag shouldbe taken into consideration.

The process of the present invention is efiective with any of the basiccopper alkali metal cyanide solutions as used in commercial andconventional copper plating. Examples of such basic copper plating bathsare given in Modern Electropl-ating, 1953 Ed., pages 194 225 inc. As iswell known, the concentration of the basic chemicals used in such bathsand the operating conditions thereof such as current density,temperature, agitation, etc. are mutually interdependent.

The following non-limiting examples illustrate the present process asapplied to copper cyanide plating baths to which the seleniumbn'ghteners aforementioned have been added. In each of the followingexamples the-temperature was about 150 F. and :1 works agitation ofabout 3 feet per minute was applied to a Hull cell panel.

Example I Copper was electrodeposited from the following aqueous bath:

CuCN 45 Free KCN 11 KOH 8 Se (from K SeSO 0.0004

The selenosulfate was added in the form of a suitably diluted aqueoussolution of good stability, such as described in my co-pendingapplication Serial No. 572,797, filed March 21, 1956, now'abandoned.

When in this bath periodically interrupted current of a cycle 15:3(seconds) was applied, a deposit ofgood brightness and uniformity wasobtained in the current density range of about 1 to 40 a.s.f. (amperesper square The polyselenide was prepared according to Organic Synthesis,vol. 18, page 27.

In this bath a periodically interrupted current of cycle 10:4 produced adeposit of good brightness and excellent uniformity over a currentdensity range of about 1 to 45 a.s.f.

Example III A copper plating bath was prepared as follows:

Free KCN 15 KOH Se (elemental) .n 0.0016

thesis, Vol.24, pages 89-93. cellent uniformity were produced over acurrent density Finely powdered selenium was added to the hot bath andthe solution was agitated for about 2 hours, when dissolution resulted.

Periodically interrupted current of a cycle 5:1 gave a deposit of goodbrightness and uniformity over a current density range of about 1 toa.s.f.

Example IV Copper was deposited from the following bath:

CuCN 45 Free" KCN ll KOH 8 Potassium tartrate 45 Se (from K SeCN) 0.0008

The selenocyanate was added in the form of a suitably diluted solution.

Application of periodically interrupted current of a cycle 20:4=produced adep'osit of good brightness and uniformity over acurrent'density range of about 1 to 40 a.s.f.

Example V Periodically interrupted current of a cycle 10:4 was appliedto a bath of the following composition:

G./l. CuCN 45 Tree KCN 11 KOH 8 So (from C H SeH) 0.05

CuCN 65 Free KCN n 15 KOH -1 9 Potassium glucbnate 12 Se (from HOC -HSeC H OH) .012

This organic selenide was made as described in Chemical Abstracts, Vol.22 (1928), 3400.

Applying a periodically interrupted current of 40:16

"gave 'a'deposit of good brightness and uniformity over a currentdensity range of about l to 55 a.s.f.

Example VII Aplating bath was prepared as follows:

7 6/1. CuCN 45 fFie KCN l1 KOH 8 Se (from K SeSO 0.0008

In this bath periodically reversed current of a cycle 20 seconds platezlsecond reversed current was applied.

Copper of good brightness and uniformity was obtained over a currentdensity range of about 1 to '45 a.s.f. In

this case the current densities apply to both the plating and thereversed current phase.

This application is a continuation-in-part of my co- .l-pendingapplication Serial No. 572,797, filed March 21,

1956; and now abandoned.

'What is claimed is:

l. A vprocess for producing decorative or protective uniform brightcopper plate in a current density range of l to 75 a.s.f., whichconsists essentially of electroplating copper at a current density of 1to 75 a.s.f. from a copper -cyanideplating-solution containing as theonly compound .which has'a brightening effect upon the copper plate'within said current density range and inorganic selenium compoundinwhich selenium has a valence of less than +4, in amount equivalent to0.00008 to 0.0032 gram per liter of selenium, while applying interruptedcurrent, where the plating phases are longer than 1 second and shorterthan about 60 seconds and the interruption phases reduce polarizationwith removal of less than of the deposit of the plating phases.

2. A process as set forth in claim 1 wherein the interruption phasesemploy a current density of from zero to 100% of that of the platingphases.

3. A process for producing decorative or protective uniform brightcopper plate in a current density range of l to 75 a.s.f., whichconsists essentially of electro-plating copper at a current density of 1to 75 a.s.f. from a copper cyanide plating solution containing as theonly compound which has a brightening effect upon the copper platewithin said current density range an inorganic selenium compound inwhich selenium has a valence of less than +4, in amount equivalent to0.00008 to 0.0032 gram per liter of selenium, while applyingperiodically interrupted current where the plating phases are longerthan about 1 second and shorter than about 60 seconds and where theinterruption phases amount to at least 10% of the plating phase time.

4. A process as set forth in claim 3 wherein the inorganic seleniumcompound is an alkali metal selenocyanate.

5. A process as set forth in claim 3 wherein the inorganic seleniumcompound is an alkali metal selenosulfate.

6. A process as set forth in claim 3 wherein the inorganic seleniumcompound is an alkali metal selcnide.

7. A process for producing decorative or protective uniform brightcopper plate in a current density range of l to 75 a.s.f., whichconsists essentially of electroplating copper at a current density of 1to 75 a.s.f. from a copper cyanide plating solution containing as theonly compound which has a brightening effect upon the copper platewithin said current density range. an inorganic selenium compound inwhich selenium has a valence of less than +4, in amount equivalent to0.00008 to 0.0032 gram per liter of selenium, while applying aperiodically reversed current where the plating phases are longer thanabout 1 second and shorter than about seconds, and where the reversedphase employs less than 10% of the coulombs of the plating phase.

8. A process as set forth in claim 7 wherein the inorganic seleniumcompound is an alkali metal selenocyanate.

9. A process as set forth in claim 7 wherein the inorganic seleniumcompound is an alkali metal selenosulfate.

10. A process as set forth in claim 7 wherein the inorganic seleniumcompound is an alkali metal selenide.

References Cited in the file of this patent UNITED STATES PATENTS2,701,234 Wernlund Feb. 1, 1955 2,737,485 Overcash et a1 Mar. 6, 19562,770,587 Ostrow Nov. 13, 1956 2,814,590 Portzer et a1. Nov. 26, 19572,854,389 Boelter Sept. 30, 1958 FOREIGN PATENTS 1,137,920 France Jan.21, 1957 1,123,209 France June 4, 1956

1. A PROCESS FOR PRODUCING DECORATIVE OR PROTECTIVE UNIFORM BRIGHTCOPPER PLATE IN A CURRENT DENSITY RANGE OF 1 TO 75 A.S.F., WHICHCONSISTS ESSENTIALLY OF ELECTROPLATING COPPER AT A CURRENT DENSITY OF 1TO 75 A.S.F. FROM A COPPER CYANIDE PLATING SOLUTION CONTAINING AS THEONLY COMPOUND WHICH HAS A BRIGHTENING EFFECT UPON THE COPPER PLATEWITHIN SAID CURRENT DENSITY RANGE AN INORGANIC SELENIUM COMPOUND INWHICH SELENIUM HAS A VALENCE OF LESS THAN +4, IN AMOUNT EQUIVALENT TO0.00008 TO 0.0032 GRAM PER LITER OF SELENIUM, WHILE APPLYING INTERRUPTEDCURRENT, WHERE THE PLATING PHRASES ARE LONGER THAN 1 SECOND AND SHORTERTHAN ABOUT 60 SECONDS AND THE INTERRUPTION PHASES REDUCE POLARIZATIONWITH REMOVAL OF LESS THAN 10% OF THE DEPOSIT OF THE PLATING PHASES.